There are various standard optical telescopic instrument systems in general industrial use for the purpose of establishing a precise coordinate reference measurement and alignment capability for the economic construction of very large fixtures and equipments. These systems have been devised to set up precise horizontal and vertical planes that are mutually perpendicular and to provide an ability to set up or measure the distance between parallel planes within a predetermined coordinate reference system.
The positioning of fixture or equipment components is usually carried out by banking a minimum of three scales against the component machined surface, viewing the offset errors and through an iteration process of adjustment and sighting bring each component to the desired location within the coordinate reference geometry. A similar technique and special apparatus for measuring the straightness of a lumber cut is described in U.S. Pat. No. 4,085,512 to Bod et al.
An undesirable aspect of the above-described scheme is that each of three scales must be checked whenever a position adjustment of the fixture is necessitated, involving re-aiming and re-focusing the telecope for each scale until the proper alignment is observed.
An early surveyor's target for a leveling rod is described in U.S. Pat. No. 405,058, issued to Thompson. The Thompson target is designed to eliminate a cosine error scaling in a linear displacement measurement from a single point on a surface. This is accomplished by aligning a centerline on a pair of angularly disposed target halves with the cross hairs of a sighting telescope. While requiring only a single sighting to sense whether the linear displacement scale is properly aligned, the Thompson target is not designed to sense the attitude or angular orientation of the surface on which the rod rests.
In general, the positioning of any fixture requires control over both angular rotation about and linear displacement along each of three axes in a reference coordinate system. It is desirable, then, to sense misalignment in as many as possible of six degrees of spatial freedom in a single telescope sighting.